Three-dimensional magnetic Fe1.3Ni0.7P nanorod assemblies (gels and aerogels) were prepared by oxidation of Fe1.3Ni0.7P nanorods capped with 11-mercapto­undecanoic acid (MUA) and 1-dodecanethiol (DDT), and their magnetic properties were evaluated in comparison to discrete nanorod precursors and in the context of the interfacial chemical characteristics associated with the ligand group employed. Treatment of MUA- and DDT-capped Fe1.3Ni0.7P nanorods with hydrogen peroxide results in gels that largely retain the structural (as assessed by powder X-ray diffraction) and morphological (as revealed by transmission electron microscopy) features of the nanorods in a pore–matter integrated network. However, aerogels created from MUA-capped particles undergo dispersion upon treatment with ethylenediamine­tetraacetic acid (EDTA), whereas those produced from DDT-capped particles remain unchanged. Likewise, reductive annealing resulted in cleavage of the aerogel produced from DDT-capped particles, whereas the aerogels produced from MUA-capped particles remained intact, if more compact, as reflected in a ca. 50% decrease in surface area. The data are consistent with distinct interfacial chemistry in the two networks: amorphous phosphite/phosphate olation linkages in aerogels produced from DDT-capped particles vs a combination of phosphite/phosphate olation linkages and metal-ion cross-linking of pendant carboxylates in aerogels produced from MUA-capped particles. The superparamagnetic blocking temperature (T B) of discrete Fe1.3Ni0.7P nanoparticles, and the aerogels made from 11-MUA and 1-DDT exchanged nanoparticles, all appear in the range 30–40 K, and the field-dependent magnetization profiles (M–H) exhibit minimal hysteresis at 50 K with coercivity (H C) values ∼10 Oe. Thus, the Fe1.3Ni0.7 x P nanoparticles comprising the aerogel network behave independently, as expected for a disordered 3-D assembly, and the magnetic aerogels can be regarded as a summation of their individual components.

Transition metal phosphides (TMPs) are a highly investigated class of nanomaterials due to their unique magnetic and catalytic properties. Although robust and reproducible synthetic routes to narrow polydispersity monometallic phosphide nanoparticles (M2P; M = Fe, Co, Ni) have been established, the preparation of multimetallic nanoparticle phases (M2–x M′ x P; M, M′ = Fe, Co, Ni) remains a significant challenge. Colloidal syntheses employ zero-valent metal carbonyl or multivalent acetylacetonate salt precursors in combination with trioctylphosphine as the source of phosphorus, oleylamine as the reducing agent, and additional solvents such as octadecene or octyl ether as “noncoordinating” cosolvents. Understanding how these different metal precursors behave in identical reaction environments is critical to assessing the role the relative reactivity of the metal precursor plays in synthesizing complex, homogeneous multimetallic TMP phases. In this study, phosphorus incorporation as a function of temperature and time was evaluated to probe how the relative rate of phosphidation of organometallic carbonyl and acetylacetonate salt precursors influences the homogeneous formation of bimetallic phosphide phases (M2–x M′ x P; M, M′ = Fe, Co, Ni). From the relative rate of phosphidation studies, we found that where reactivity with TOP for the various metal precursors differs significantly, prealloying steps are necessary to isolate the desired bimetallic phosphide phase. These insights were then translated to establish streamlined synthetic protocols for the formation of new trimetallic Fe2–x–y Ni x Co y P phases.

Colloidal synthesis of MnAsxSb1−x (x = 0.1–0.9) results in As-deficient crystalline cores with amorphous oxide shells. Magnetic studies reveal size and phase dependent magnetic properties, with a clear relationship between core size and coercivity.